1. Field of the Invention
The present invention relates in general to wastewater treatment methods and systems, and more specifically to a method and system for controlling oxygen utilization when treating wastewater using a series of closed-tanks and an oxygen-enriched gas (high-purity oxygen). Wastewaters treated with closed-tanks and high-purity oxygen can, for example, be municipal sewage, slaughter house waste, waste from petrochemical and paper plants, or biological sludges generated from wastewater treatment. These wastewaters must be treated to reduce levels of organic matter, nitrogenous compounds, phosphorous, and other materials considered as pollutants.
2. Description of the Related Art
A number of different methods have been employed for wastewater treatment. Many of these methods involve biochemical oxidation by aerobic bacteria to convert various pollutants to other forms of matter. A common example is the activated sludge process which utilizes an aeration tank or reactor and a setting tank or clarifier. Wastewater is mixed with a concentrated solution of aerobic bacteria (sludge) in the aeration tank where biochemical oxidation takes place. The mixture (effluent from the aeration tank) is then delivered to the settling tank where the bacteria settle and serve as the concentrated solution (sludge) for return to the aeration tank. Treated water from the settling tank exits generally at the top of the tank and is released for discharge or subsequent treatment.
In order for the biochemical oxidation reaction to take place, oxygen must be supplied to the mixture (mixed liquor) in the aeration tank. The type and rate of reactions are dependent upon the amount of oxygen available for use by the bacteria. Oxygen is usually made available to the bacteria in the form of dissolved oxygen (DO) by dissolution of oxygen into the liquor from the aerating gas above the aeration tank. Air, which has an oxygen partial pressure of about 160 millimeters of Mercury (mm Hg), is the most common source of gas for dissolution of oxygen into the liquor when using open-top aeration tanks. High-purity oxygen can also be used economically as the aerating gas, but the aeration tank must usually enclose the aerating gas space above the liquor in order to achieve a high oxygen utilization efficiency. Since the partial pressure of oxygen in the enclosed gas space is normally above 300 mm Hg, the natural driving force for transferring oxygen from the gas space to the mixed liquor is much higher than for open air systems. When high-purity oxygen is used in this manner with a closed-tank aeration system, the process is usually termed the oxygen activated sludge process.
Several modifications of the oxygen activated sludge process have been proposed that utilize closed-tank aeration systems. These versions include those disclosed in U.S. Pat. No. 3,547,812; U.S. Pat. 3,547,815; U.S. Pat. No. 3,725,258; and U.S. Pat. No. 4,442,005. All of these systems require that the pressure in the enclosed gas space be maintained above existing atmospheric pressure for the purpose of forcing oxygen to flow through the system and for positively venting exit gasses to the atmosphere. Venting is necessary to prevent the buildup of nitrogen, carbon dioxide, and other gasses which reduce the partial pressure of oxygen in the gas space and thus reduce the oxygen transfer efficiency of the system. For reasons that will become later apparent and for convenience, only the operation of the conventional oxygen activated sludge process as generally disclosed by U.S. Pat. No. 3,547,815 will be described.
In the conventional oxygen activated sludge process, the aeration tank or reactor is usually separated into two or more closed-tank chambers which operate in series. Wastewater and sludge returned from the settling tank are usually input to the first chamber and the mixed liquor flows through subsequent chambers in the series and eventually to the settling tank or clarifier. High-purity oxygen gas containing from 60 to 99% oxygen by volume is normally input to the gas space of the first chamber, and it flows through the gas space of subsequent chambers concurrent with the mixed liquor. Some versions of the process allow wastewater, return sludge, and high-purity oxygen to be input to any or all chambers of the reactor. Various types of mixing devices are used to enhance oxygen transfer to the mixed liquor of each chamber where dissolved oxygen is consumed by the biological reactions. The most common devices are surface aerators that use a multi-bladed impeller located at the gas-liquid interface and gas recirculation systems that utilize a sparger submerged in the mixed liquor. The mixing devices create a large gas-liquor interfacial area to enhance dissolution of oxygen, and also stir the mixed liquor so that the bacterial solids remain in suspension uniformly throughout each chamber.
High-purity oxygen is usually supplied to the oxygen activated sludge process form an on-site oxygen generating plant or it may be supplied directly from a commercial pipeline. The source of high-purity oxygen is delivered above atmospheric pressure to the system in a controlled manner to provide a near constant gas phase pressure in the first chamber of the reactor. The system is operated at a desired constant pressure of usually 2 to 6 mm Hg above atmospheric pressure in the first chamber. An operating pressure above existing atmospheric conditions is required is the reactor since the exit gases from the last chamber are vented to the atmosphere, and a differential gas phase pressure from the first to the last chamber is needed to force the high-purity oxygen through the series of chambers. The exit gas flow is usually adjusted manually or automatically by means of a valve on the exit gas line in an attempt to provide approximately 50% oxygen in the exit gas (vent gas).
The key to the success of the conventional oxygen activated sludge process is the relatively high oxygen utilization efficiency provided by the oxygen dissolution system. Past attempts to use high-purity oxygen failed to be competitive with air processes because much of the oxygen supplied was usually lost and not used by the biological reactions. Since wastewater flow rates and pollutant concentrations vary significantly with time, the oxygen demand by the biological reactions is also highly variable. Thus, to obtain high utilization of oxygen, the oxygen feed rate must be varied to match the demand. This is accomplished more effectively in the conventional oxygen activated sludge system than in prior art systems by controlling the oxygen feed rate based on the total pressure in the first chamber of the reactor. A pressure sensor in the gas phase of the first chamber monitors pressure changes and sends a signal to an oxygen feed controller. Depending on the difference between the measured pressure and the controller setpoint pressure, which is the constant pressure desired in the first chamber, the controller manipulates a control valve on the oxygen feed line as a means of controlling the supply of oxygen and maintaining the desired constant pressure (setpoint pressure). This oxygen feed control strategy is effective since as oxygen demand increases in a closed-tank system, oxygen transfer from the gas to the liquid phase increases causing a decrease in the partial pressure of oxygen in the gas phase and a drop in the total gas phase pressure. Thus, an oxygen demand increases, more oxygen is supplied and as demand decreases, the oxygen supply is reduced.
Although the strategy for controlling oxygen dissolution in the conventional oxygen activated sludge processs has allowed the use of high-purity oxygen to be economically competitive with open air systems, the overall oxygen utilization efficiency is usually less than 85% and the potential for further improvement still exists. Since the gas phase pressure is maintained above atmospheric pressure, gas leaks through cracks and pin holes in the cover of a closed-tank reactor can be significant. Oxygen losses due to leakage can exceed 10% of the oxygen feed when operating the first chamber with a 3.7 mm Hg setpoint pressure for controlling the oxygen feed rate. Even for a gas-tight reactor cover, which is practically impossible and very costly to construct, the conventional control strategy usually provides for only 80 to 85% utilization of the oxygen feed by the biological reactions. The remainder of the oxygen feed is lost through dissolved oxygen in the effluent from the reactor and by the intentional venting of exit gasses which include oxygen.
Oxygen losses associated with the conventional control strategy increase as the percentage of oxygen in the gas phase increases causing reduced oxygen utilization efficiency. Therefore, it is best to operate with a percentage of oxygen in the gas phase, and particularly the gas phase of the last chamber, as low as possible without causing dissolved oxygen (DO) levels in the mixed liquor that adversely affect the treatment efficiency. At low biochemical oxygen demand (BOD) loadings, it is possible to operate the system without adverse DO affects by manually adjusting the vent gas control valve to a fixed position to provide as average vent gas composition of less than 50% oxygen. But when BOD loadings approach design capacity of the treatment system, fixing the position of the vent gas valve opening to provide a daily average of 45% oxygen in the vent gas can result in frequent DO depletions because of extreme fluctuations in the percentage of oxygen in the gas phase of the last chamber of the reactor. Attempts have been made to maintain a constant vent gas composition by automatically adjusting the opening of the vent gas valve using a control signal based on measurement of the percentage of oxygen in the vent gas. However, computer simulations using a dynamic model and practical experience indicate that there is no significant advantage of automatic control over periodic manual adjustments of the vent gas valve. Variations in the effluent DO from the reactor and vent gas composition are practically the same as for the case of manually fixing the position of the vent gas valve. Also, DO depletions cannot be prevented at design BOD loadings when using 45% oxygen in the vent gas as a setpoint for automatic control of the vent gas valve opening.
The inadequacy of vent gas control for the conventional operating scheme is due primarily to the small vent gas flow rates which are limited by operating the reactor with a low pressure in the gas phase. The vent gas flow rate is very small relative to the gas phase volume, thus making it difficult to rapidly change the vent gas composition. If higher setpoint pressures are used for controlling the oxygen feed, better control of the vent gas composition is achieved, but oxygen losses also increase because gas phase leaks are dependent on the gas phase pressure. Thus, it is obvious that the oxygen feed and vent gas control objectives are conflicting from the standpoint of minimizing oxygen losses and very few adjustments can be made to further improve the oxygen utilization efficiency for the conventional operating method. The primary deficiencies are the need to operate the gas phase above atmospheric pressure and the inability to control the oxygen level in the vent gas and the effluent DO from the reactor.
A preliminary patentability search in class 210, subclasses 614, 623 and 627 disclosed the following patents: Bringle, U.S. Pat. No. 3,342,727; McWhirter, U.S. Pat. No. 3,547,812; McWhirter, U.S. Pat. No. 3,547,815; Spector et al, U.S. Pat. No. 3,725,258; Kirk, U.S. Pat. No. 3,983,031; Mandt, U.S. Pat. No. 4,205,047; Chen et al, U.S. Pat. No. 4,271,026; Breider, U.S. Pat. No. 4,442,005; and Friedman et al, U.S. Pat. No. 4,563,281. None of the above prior art discloses or suggest the present invention which is based upon the finding that significant improvements in oxygen utilization efficiency can be achieved when using high-purity oxygen by operating a closed-tank reactor at or below atmospheric pressure and using an exhaust apparatus to remove vent gasses from the last chamber of the reactor.